Our gym did away with the secret sister thing this year because some parents complained it cost too much(it doesn't have too!!!) and others felt bad because a secret sister gave them very nice gifts. Can't win.
Some thoughts for an 11 yo----small stuffed animals, inexpensive water bottle, candy---not healthy, but enjoyed. I found that many girls really like cards handmade by their "secret" sister. My gymmie's fav gift was a cute little journal that she could put her initials on. She keeps it in her gym bag and plans on using it again this year to write down goals for each meet.
Just remembered---bubble bath type stuff.